This invention relates generally to digital radio systems, and more particularly, to methods for simulcasting or monocasting in digital radio systems, especially private land mobile radio (LMR) systems.
Digital radio communication systems typically use a broad array of different technologies for modulation and demodulation so that information can be effectively transmitted over a radio channel. Radio communications systems have become standardized to facilitate reliable and interoperable communications in a variety of standards. For example, the Project 25 suite of standards (documented by the Telecommunications Industry Association (TIA) in TIA-102.BAAA) and numerous related standards are known. The Project 25 suite of standards provides Frequency Division Multiple Access (FDMA) channels, which are most often on channels spaced 12.5 kilo-Hertz (kHz) apart. The Project 25 standard will likely expand to encompass a Time Division Multiple Access (TDMA) standard that will also fit into channels spaced 12.5 kHz apart (herein referred to as a narrow band channel).
In order to provide extended communication coverage areas, particularly in large private radio systems, such as trunked radio systems in the 800 MHz band, simulcast channels are often implemented. Simulcast transmission, as performed with mobile radio technology, provides for identical modulation schemes to be used to communicate the same information from different locations. For example, the same information is transmitted from two or more sites (e.g., base stations) within a coverage area of the system. This information is transmitted simultaneously, synchronously, and on the same radio channel, so that radio receivers typically receive a signal composed of two or more radio signals that are incident on the receiver antenna. Accordingly, it is important for communications on the TDMA standard for Project 25 to operate reliably and effectively on simulcast channels with 12.5 kHz channel spacing.
The Project 25 suite of standards is intended to operate with digital voice communication such that the suite includes a standardized vocoder to digitize voice for transmission. The vocoder may either operate in a full rate mode, for FDMA transmissions, or a half rate mode, for TDMA transmissions, with the channel separated into two slots, for two simultaneous voice calls. The known Project 25 standards for FDMA channels define a modulation family that includes Compatible 4-level Frequency Modulation (C4FM) and Compatible Quadrature Phase Shift Keying (CQPSK) at a bit rate of 9.6 kbps (as described and defined in the TIA-102.BAAA standard). There are also proposals to define a standard for TDMA channels. The TDMA standard will likely use a half rate voice encoder to scale the bit rate for the voice information by a factor of two. However, the overall bit rate of the channel may need to increase beyond 9.6 kbps to allow a margin for switching times. When providing simulcast communications at bit rates higher than 9.6 kbps using the known communications standards, less than acceptable transmission quality may result. Moreover, the simulcast range when using the known standards at 9.6 kbps also may be less than desired.
The current Project 25 standard includes the measurement of a figure of merit for simulcast operation that is known as the signal delay spread capability, or more simply and often referred to as the delay spread. The signal delay spread capability measures the performance of the receiver when the receiver receives two independently faded signals that are delayed relative to each and which are identically modulated, as would be expected in a simulcast system. The signal delay spread capability measures the maximum delay that the receiver may tolerate while maintaining a usable bit error rate (BER). In the Project 25 standard, the signal delay spread capability is measured according to the TIA-102.CAAA standard in clause 2.1.6.2 and the specification for the delay spread is given in the TIA-102.CAAB standard in clause 3.1.6 at 50 microseconds for 5% BER. Accordingly, it is desirable to improve this performance.